In the past, diisononyl phthalic acids (DIN, DIAN, etc.) have been used as a plasticizer for polyvinyl chloride (PVC) resins, etc. (e.g., see Japanese Patent Publication (A) No. 2006-513298). However, while these plasticizers are used as a shape stabilizer for PVC, they are not known to be compounded into diene-based rubbers.